This project will significantly impact biology and medicine by better defining molecular mechanisms that regulate spermatogonial stem cell development. Each specific aim is centered on defining the structure, function and testicular endocrinology of a family of survival, proliferation and differentiation factors, termed neuregulins. Neuregulins comprise a large family of ligands for transmembrane receptors in the erythoblastoma virus B (ErbB) family of tyrosine kinases. Preliminary studies show that neuregulins and their receptors are expressed together on undifferentiated spermatogonia, and that neuregulin-like factors are required for formation of aligned spermatogonia in culture. The proposed studies are based on these findings and will represent a new area of investigation in reproductive biology. The goal of Specific Aim 1 is to define the structures of neuregulin and ErbB family members expressed by undifferentiated spermatogonia in culture. Ligand-receptor pairs identified will then be used to determine mechanisms by which neuregulins regulate spermatogonial development in culture. Specific Aim 2 will focus on defining the spatial and temporal expression of one neuregulin receptor, termed ErbB3, in the rat testes. Preliminary studies show that expression of ErbB3 is highly restricted to type A-single spermatogonia within rat testes. Because type A-single spermatogonia are considered stem spermatogonia, and because specific molecular markers for these cells have yet to be defined, ErbB3-expressing spermatogonia will be tested for their ability to function as spermatogonial stem cells in chimeric rodent testes, as Specific Aim 3. The proposed project should have a positive impact on human health; elucidating molecular mechanisms that regulate spermatogonial development may well lead to treatments and cures for male infertility, provide new targets for contraceptive development, assist in the preservation of endangered species, afford alternative methods to produce transgenic animals valuable in medicine, and potentially allow for selection against harmful genetic defects in the male germline.